1. Field of the Invention
The present invention relates to devices and methods for joining abutting sections of multi-cell conduit.
2. Description of Related Art
Multi-cell conduit, and in particular quadri-cell conduit having four interior cells, has been used in various applications in the prior art. Because of this, there are also at least several known methods and devices for joining abutting sections of this conduit to hold the internal cells of one conduit section in alignment with the associated cells of the adjoining section.
The configuration of a typical quadri-cell conduit section has a single external wall having a generally square cross-sectional shape. Within this external wall is a web, or cross-shaped piece, usually made of the same material, each of the ends of the cross being attached to a different side of the external wall.
When the conduit is structurally fairly rigid, one of the more common ways to join and hold sections together is to use a sleeve which will fit snugly around the exterior perimeter of the ends of the two adjoining conduit sections. The adjoining ends of the sections are inserted into opposite ends of a sleeve, and the sections may be permanently secured to the sleeve by cementing the interior of the sleeve to the exterior of the conduit. A sleeve of this type may also be a two piece device held in place tightly by clamps or similar fastening means.
Another known approach, shown in U.S. Pat. No. 722,249, for example, is to provide external notches or grooves into which alignment pins may be placed which will cause two sections to become aligned. This method, as described in the patent, is to be followed by the application of cement to the entire exterior of the joint to seal it from water.
U.S. Pat. No. 3,133,753 issued to Goodman, et al discloses a further approach. In one of the embodiments disclosed in this patent, each conduit section has a male and female end section. These specially configured end portions allow one end of a section to be inserted into an appropriately configured end of a second section of conduit. This joint takes on the appearance of interlocking fingers, which are subsequently pinned together with a special anchor pin.
In multi-cell conduit systems it is often desirable to isolate individual cells, and make them air tight. The aforenoted joining methods effectively join the exterior walls to one another, and will generally protect, or isolate, the joint from the external environment, but the individual internal cells are only isolated to the extent that the edges of the webs of the sections abut each other or come close to abutting each other. This is generally inadequate if it is desired to isolate individual cells from each other.
A primary modern use of quadri-cell conduit is for the underground carrying of fiber-optic, electrical, and/or telephone cables. Systems using multi-cell conduit can be designed to facilitate the addition of more cables for increased transmission capacity, either planned or unexpected. When the initial installation is made, generally only one or two of the four cells of a quadri-cell conduit string will be used to carry cable immediately and a sufficient number of conduit runs are laid so that there will be a sufficient number of spare or unutilized cells. Thus, when expansion is desired, roadways, sidewalks and land will not have to be torn up because the additional cables may be run through in the unutilized cells of the conduit previously installed.
It can be imagined that during initial installation, a cable may be fed incrementally through short runs of conduit. However, once the entire length of a conduit run has been laid and buried underground, any new installation would required the cable to be fed through the entire length all at once. One way in which this is accomplished in the industry is by propelling a line from one end of the run through to the opposite end of the run. The line is provided with a leader which covers all or substantial portion of the cross-section of a cell. The propulsion, provided by either a compressed air source or by controlled explosion, must push the line leader through the length of the cell, carrying the line with it. The cable to be installed is then attached and pulled through by the line. This is a preferred method even for initial installations of conduit and cable.
Because these conduit runs span great lengths, maintaining the pressure in the cell to drive the line through is critical to the success of the installation. The previously mentioned methods and devices for joining sections of conduit together do not generally provide a substantially airtight isolation of the individual cells from each other, and therefore a loss of pressure is likely to be experienced in a particular cell at each joint between sections of conduit. The presssure losses in long runs having many joints may be substantial enough that the line leader will not be propelled through the entire length, but will stop short of the opposite end of the run instead. This will tend to limit the maximum length over which a successful installation can be made.
It is therefore a principal object of the present invention to provide a multi-cell conduit system wherein a plurality of conduit sections are joined to each other in an end to end configuration, the system and joint design providing improved isolation of the individual cells from one another, as well as from the external environment, by use of cell inserts.
It is a further important object of the present invention to provide a method for making a joint and joining abutting sections of multi-cell conduit using cell inserts, which will facilitate installation of long runs of conduit containing many sections.
It is a further important object of the present invention to provide a system for joining, in an end-to-end manner, sections of substantially square cross-sectioned conduit containing four individual cells, wherein the four cells of each conduit section are symmetrical with respect to a 90.degree. rotation about the longitudinal axis of the section, and the cell inserts are all identical and interchangeable.
It is a further important object of the present invention to provide a system for joining sections of multi-cell conduit which allows increased flexibility and ease of field installation operations.
It is another important object of the present invention to provide a simple and inexpensive way of making a joint which does not require an unduly burdensome inventory control to ensure all piece parts for making a joint are available in the field.